docs: Embellish explanation of piezo buzzer types.

Expand the introduction to piezo buzzers to explain that there are two
types with slightly different uses.  It is important that the student/user
be using an active buzzer for the example code to work.

Also add text stating that a passive piezo buzzer could be used as an
alternative to a speaker and updated the warning to say that active piezo
buzzers shouldn't be used.

docs/tutorials/io.rst

@@ -45,14 +45,19 @@ when you plug in circuits and other devices via the pins.
 Bleeps and Bloops
 +++++++++++++++++
 
-The simplest thing we can attach to the device is a Piezo buzzer. We're going
-to use it for output.
+The simplest thing we can attach to the device is a Piezo buzzer. There are two 
+types of piezo buzzers. The simplest type to use are called active buzzers. 
+Active buzzers contain an oscillator that produces a tone at a predetermined
+pitch when a current is passed through them.  Passive buzzers require an 
+oscillating current to be passed through them to produce a tone at the frequency
+of the oscillation.  In short, active buzzers are simple to use but produce only
+one tone, while passive buzzers are slightly more complicated to use but can 
+produce a variety of tones.
 
 .. image:: piezo_buzzer.jpg
 
-These small devices play a high-pitched bleep when connected to a circuit. To
-attach one to your BBC micro:bit you should attach crocodile clips to pin 0 and
-GND (as shown below).
+We're going to use an active piezo buzzer for output. To attach one to your BBC 
+micro:bit you should attach crocodile clips to pin 0 and GND (as shown below).
 
 .. image:: pin0-gnd.png
 

docs/tutorials/music.rst

@@ -3,15 +3,16 @@ Music
 
 MicroPython on the BBC micro:bit comes with a powerful music and sound module.
 It's very easy to generate bleeps and bloops from the device *if you attach a
-speaker*. Use crocodile clips to attach pin 0 and GND to the positive and
-negative inputs on the speaker - it doesn't matter which way round they are
-connected to the speaker.
+speaker or passive piezo buzzer*. Use crocodile clips to attach pin 0 and GND
+to the positive and negative inputs on the speaker/buzzer - it doesn't matter
+which way round they are connected to a speaker, but a piezo buzzer may be
+polarized (check for a "+" marking the positive terminal).
 
 .. image:: pin0-gnd.png
 
 .. note::
 
-    Do not attempt this with a Piezo buzzer - such buzzers are only able to
+    Do not attempt this with an active Piezo buzzer - such buzzers are only able to
     play a single tone.
 
 Let's play some music::